Add new TCSSBuilder.EnsureSelector method

This added because attempting to add an existing selector raises an
exception and reading a non-existant selector returns a null pointer.
This makes it hard to write bug-free code without checking for the
existence of a selector. It's been annoying me for years!

Author: delphidabbler

Src/UCSSBuilder.pas

@@ -83,6 +83,12 @@   TCSSBuilder = class(TObject)
         @param Selector [in] Name of new selector.
         @return New empty selector object.
       }
+    function EnsureSelector(const Selector: string): TCSSSelector;
+      {Returns selector object with given name or adds a new selector with the
+      given name if no such selector exists.
+        @parm Selector [in] Name of selector.
+        @return Reference to new or pre-existing selector.
+      }
     procedure Clear;
       {Clears all selectors from style sheet and frees selector objects.
       }
@@ -204,6 +210,18 @@ destructor TCSSBuilder.Destroy;
   inherited;
 end;
 
+function TCSSBuilder.EnsureSelector(const Selector: string): TCSSSelector;
+  {Returns selector object with given name or adds a new selector with the given
+  name if no such selector exists.
+    @parm Selector [in] Name of selector.
+    @return Reference to new or pre-existing selector.
+  }
+begin
+  Result := GetSelector(Selector);
+  if not Assigned(Result) then
+    Result := AddSelector(Selector);
+end;
+
 function TCSSBuilder.GetSelector(const Selector: string): TCSSSelector;
   {Read access method for Selectors property. Returns selector object with given
   name.